This invention relates to an exhaust gas recirculation system for an internal combustion engine. It relates particularly to a recirculation system which recycles exhaust gases into the induction system for the primary purpose of reducing the oxides of nitrogen emitted from the exhaust.
The amount of nitrogen oxides emitted from an internal combustion engine is a function of combustion temperature. Relatively high combustion temperatures, such as occur during engine operation at partially open throttle at level cruise and at relatively high speeds, produce large amounts of nitrogen oxide emissions.
Such emissions are not critical during idling because the rate of fuel combustion is low enough that the combustion temperatures are relatively low.
The engine combustion temperatures normally increase with engine load and with the rate of acceleration at any speed.
However, engine operation at wide open throttle normally results in enough fuel enrichment by the power valve that the combustion temperatures are lowered enough to produce only acceptable amounts of nitrogen oxide emissions. Recycled exhaust gas flows may therefore be discontinued or reduced to a minimum during operation at wide open throttle with the power valve on.
Recycling 5 to 25 percent of the total exhaust gases through the engine (depending on load and power demand) will usually reduce combustion temperatures to levels that do not produce unacceptable amounts of nitrogen oxide emissions. For example, recycling about fifteen percent of the total exhaust gases during partially open throttle will usually produce the desired result.